The present invention relates to the field of structural panel fasteners and, more particularly, pertains to the use of a structural grommet to reinforce the mounting hole in a panel of composite material and a fastening assembly for use therewith in attaching the composite panel to a substructure.
Structural panels made of composites of light-weight materials are being used more extensively in many applications requiring high strength and relatively light weight. For example, the high tensile strength and stiffness of structural panels made of graphite fibers disposed in a thermoplastic resin matrix make these panels extremely useful in aerospace applications where a high strength-to-weight ratio is important. However, these composite panels tend to be very brittle and conventional fastening means, used successfully in other types of structural panels, may fracture, chip, or crush composite panels when applied in their usual manner.
One component of a fastener assembly used extensively in various light-weight structural panels is a reinforcing grommet. A grommet is a hollow cylindrical insert which is installed in a hole in a panel to reinforce the opening for receipt of a fastener element, such as a bolt. A grommet typically has an enlarged head by which it is held in one face of the panel and an extended cylindrical opposite end which is flared radially to engage the other panel face surrounding the hole to secure the grommet in place.
When a conventional flared grommet is installed and used in a structural panel of composite material, it has been found that damage to the panel may occur in several ways. First, the radial flaring of the grommet may crush the panel corner at the junction of the surfaces of the hole and the panel, which can propagate cracks and cause high stress concentrations. Second, the free end of the flared portion is, turned radially outwardly and back against the panel surface when the grommet is installed, resulting in sharp line contact which may chip or crack the panel surface and lead to immediate failure or subsequent failure when the fastener is loaded. Finally, when the panel and substructure are separated with a gasket material, which material must obviously not occupy the space immediately surrounding the flared grommet end, and the panel and substructure are drawn together by the fastener disposed in the grommet, the unsupported load imposed on the panel through the grommet may cause the panel to fracture and fail.